1. Field of the Invention
The present invention relates to a method for preparing lower olefins from methanol and/or dimethyl ether, and more specifically it relates to a method for preparing lower olefins such as ethylene and propylene from methanol and/or dimethyl ether by the use of catalyst with a high selectivity and is stable for a long term, the used catalyst being an alkaline earth metal-containing aluminoborosilicate modified with an alkaline earth metal compound.
2. Description of the Prior Art
In recent years, the supply of crude oil is unstable, and particularly in countries which depend largely on foreign countries for the oil, an effective utilization of coals and natural gases is taken up as an important theme. It is now desired to establish an industrially synthetic method of preparing organic compounds such as olefins, paraffins and aromatic compounds by using, as a raw material, methanol which can be obtained from methane, CO and the like.
Nowadays, low-cost ethylene has been produced from natural gases, and it can be presumed that ethylene and ethylene derivatives will be manufactured excessively in the near future. As a result, if the manufacture of the ethylene is controlled, the manufacture of propylene which is produced together with the ethylene will also be curtailed, and in consequence, the shortage of propylene in the future can be supposed. For this reason, it is sought to develop a method for manufacturing lower olefins such as ethylene and propylene simultaneously and stable in high yield.
It is known in the art that silica, alumina, crystalline aluminosilicate and the like have been heretofore employed as catalysts for hydrocarbon conversion. The crystalline aluminosilicate contains pores or tunnels therein having a certain diameter, depending on its kind, and therefore it has the shape selectivity that molecules alone which satisfy specific conditions are selectively adsorbed out of various molecules which are mixed. Accordingly, such a crystalline aluminosilicate is also called a molecular sieve generally.
In the 1970's, Mobil Oil Co., Ltd. has developed a ZSM-5 type zeolite catalyst as the shape selectivity catalyst which can manufacture hydrocarbons containing high-quality gasolines as the main components from methanol or dimethyl ether. This zeolite is different from conventional ones, because it permits freely controlling a molar ratio of SiO.sub.2 /Al.sub.2 O.sub.3 and has an excellent property such as a remarkably high heat resistance. By utilizing the features of the new zeolite, it is possible to prepare a product containing lower olefins as main components through a conversion reaction of methanol and/or dimethyl ether. For example, according to the specification of West German Pat. No. 2935863, it is known that the active type zeolite (H-ZSM-5) of SiO.sub.2 /Al.sub.2 O.sub.3 =35 to 1600 (molar ratio) can produce lower olefins (carbon number=2 to 4) in a maximum yield of 70.1% by weight in a methanol conversion reaction within a temperature range of 350.degree. to 600.degree. C. It is described in examples of the West German Patent that an optimum composition of the ZSM-5 type zeolite and the reaction temperature are SiO.sub.2 /Al.sub.2 O.sub.3 =298 to 500 (molar ratio) and 550.degree. C., respectively. Accordingly, in order to form a hydrocarbon containing the lower olefins as main components from methanol and/or dimethyl ether, it is definitely advantageous that the reaction temperature is as high as possible, but the methanol conversion reaction at such a high temperature as about 550.degree. C. will often lead to a rapid deterioration in the catalyst, for example, even the ZSM-5 type zeolite catalyst having a high heat resistance. Thus, for the purpose of manufacturing the lower olefins at a temperature of 500.degree. C. or more from a raw material such as methanol and/or dimethyl ether in a high yield for a long term without involving a rapid decline in the catalyst, it is necessary to develop a novel zeolite by which benzene, toluene and xylene (B. T. X.) which can be supposed as coke precursors are less produced and which does not bring about an activity decline easily at a high temperature of 550.degree. C. or more.
From such a viewpoint, the inventors of the present application have intensively researched to develop a catalyst which scarcely deteriorates in the conversion reaction of methanol and/or dimethyl ether at a high temperature of 500.degree. C. or more at which the production of the lower olefins is advantageous, and as a result, it has been found that a finely crystalline alkaline earth metal-containing aluminoborosilicate (hereinafter referred to simply as aluminoborosilicate on occasion) which has been synthesized by adding boron and a salt of an alkaline earth metal at the time of its synthesis is suitable for this object, is remarkably excellent in selectivity of the lower olefins, and provides a less production of B. T. X. This fact has been already disclosed (Japanese Patent Application No. 245435/1985). However, even in the case of this catalyst, its activity deteriorates owing to the deposition of carbon onto the catalyst, as reaction time elapses, and thus it cannot withstand long-term use sufficiently.
The present inventors have continued the research with the intention of prolonging the life of the catalyst, and in the end, they have found that when the above mentioned alkaline earth metal-containing aluminoborosilicate is further modified with an alkaline earth metal compound such as a salt, an oxide or a hydroxide of an alkaline earth metal, an extremely noticeable improvement is made unexpectedly. The present invention has been completed on the basis of this finding.